Microlithography is used for producing microstructured components such as, for example, integrated circuits or LCDs. The microlithography process is carried out in what is called a projection exposure apparatus, which includes an illumination device and a projection lens. The image of a mask (reticle) illuminated via the illumination device is in this case projected via the projection lens onto a substrate (for example a silicon wafer) coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection lens, in order to transfer the mask structure to the light-sensitive coating of the substrate.
In a projection exposure apparatus designed for EUV (i.e. for electromagnetic radiation with a wavelength below 15 nm), mirrors are used as optical components for the imaging process because of the unavailability of light-transmissive materials. In the illumination device of a microlithographic projection exposure apparatus designed for operation in the EUV range, in particular the use of facet mirrors in the form of field facet mirrors and pupil facet mirrors as focusing components is known for example from DE 10 2008 009 600 A1. Such facet mirrors are constructed from a multiplicity of individual mirrors or mirror facets, which each may be designed to be tiltable by way of flexure bearings for the purposes of adjusting, or else for realizing, specific illumination angle distributions.
In a projection exposure apparatus, when manipulating the position of an element, such as for example a mirror, it is possible in principle for the respective position of a mirror to be measured via a position sensor and set to the desired value via a controller using an actuator. However, this fundamentally entails the problem that, on the basis of Newton's principle of “actio=reactio”, every force exerted by an actuator on an element, such as for example the respective mirror, is accompanied by a force of reaction of the same amount acting in the opposite direction.
To illustrate this, FIG. 1 shows the breakdown of an open control circuit into an action path and a reaction path. According to FIG. 1, a position controller 11 generates a signal for an actuator 12, one component of the force that is produced by the actuator 12 being transferred in the action path to an element (for example mirror) 13 to be positioned and the other component of the force that is produced by the actuator 12 being transferred in the reaction path to mechanical components (for example force or sensor frames) located therein. This may involve the excitation of resonances—typically weakly damped resonances—of the mechanical components located in the reaction path, which at the desired bandwidth can in turn destabilize the position control circuit.
Known ways of overcoming this include the use of reaction masses for the respective actuators and the creation of a sensor frame that is mechanically decoupled from a carrying frame.
Reference is made merely by way of example to U.S. Pat. No. 6,788,386 B2 and WO 2012/152520 A1.